But, I get an error under the second "generateElementDoc" in "Documentation.mtl B" which is saying
"No public template 'generateElementDoc' found." even if the package containing Documentation.mtl A
is exported.

Where am I wrong ?

The last question is : since "plugin A" which implements the action doesn't know "plugin B", I will have to
use "dynamic template" extension for declaring "Documentation.mtl B" ?

First of all, I think you would need to have the uri of your two metamodels in the module B to make it work, and if you want to have no dependencies in your first generator to the second generator, you have to use the extension point for the dynamic overriding.